Quad-40 

4ch Tube Power Amplifier

Summary

This is the quad-40 tube audio amplifier which anchors, figurartively and literally, my tube home theater system.  In designing this amplifier I have attempted to squeeze as much power as I can out of a pair of EL34 pentodes while still maintaining high fidelity performance resulting in a bandwidth of 20 Hz to 25 KHz and a THD of 0.45 % while achieving a peak output power of approximately 80 watts RMS per channel (it started with only 40 watts per channel).  

These days I run a lower bias, well into the class B regime, and likely squeeze even more power out of it because it is powering speaker pairs throughout the warehouse at my place of work.

Publications

• 'Keep Those Filaments Lit, Design Your Own Vacuum Tube Audio Equipment.' Hackaday, June 4, 2014.

• G. L. Charvat, “Vacuum tube home theater system (part 2), vacuum tube power amps for home theater,” Audio Express Magazine, June 2012.

• Make Magazine Blog, ‘How to design a  vacuum tube amplifier.’

• G. L. Charvat.  “Design and fabrication of high-fidelity vacuum tube audio-frequency power amplifiers,”  MIT Haystack Observatory, February 3, 2010.

Engineering Notes and Schematics

Caution!  High voltage.  Do not attempt to build anything shown here unless you have taken proper high voltage training.  The high voltages present in these tube designs are deadly.

Quad-40 amplifier notes

Biasing notes

Bode plot analysis & loop compensation network notes.

Bode plots and loop compensation network measurements & calculations MATLAB script.

0.45 % measured Total Harmonic Distortion.  Total Harmonic Distortion (THD) tests and analysis notes.